Deep channels in the Cenomanian–Danian Chalk Group of the German North Sea sector: Evidence of strong constructional and erosional bottom currents and effect on reservoir quality distribution

Abstract

Reflection seismic lines from the Schillgrund High in the German North Sea sector show that the sea-floor deposition of the Upper Cretaceous–Danian Chalk Group was extremely irregular. The Cenomanian–Campanian part of the group is up to 600 m (1968 ft) thick and is dominated by pelagic, relatively horizontally deposited chalk. It was tilted toward the southwest close to the Campanian–Maastrichtian boundary time and was overlain by up to 800 m (2625 ft) of onlapping Maastrichtian–Danian chalk that was eroded and reworked by powerful synsedimentary bottom currents. A north–south to north-northwest–south-southeast–trending slope-parallel multistorey channel system, branching toward the northwest into smaller, laterally directed arms, was developed in Maastrichtian–Danian times. It was more than 200 km (124 mi) long, up to 15 km wide (9 mi), and 200 m (656 ft) deep. The main channel system was eroded by powerful contour-parallel bottom currents, interpreted as having flowed toward the southeast based on the depositional geometry of the channel fill. Successive channel axes were systematically shifted downslope, leaving a series of slightly offset, nested slope-parallel channels. The channels were filled laterally, obliquely, or longitudinally by chalk drifts. Sediment gravity flows were a factor locally but were not integral parts of the depositional system. Erosion, winnowing, sorting, and redeposition by the bottom currents are interpreted to have resulted in channel-parallel porosity-permeability zones. The seismic data indicate that the chalk adjacent to the main channels retained high porosity calculated from logs in an exploration well drilled in 2005. The chalk is of excellent reservoir quality with effective porosity averaging 37%, reaching 50%, and some levels are completely unconsolidated. The influence of powerful syndepositional bottom currents on the anisotropy of reservoir properties of chalk is only now beginning to be realized, chalk having traditionally been considered a monotonous and uniform rock type.